The arc tube for high pressure metal vapor discharge lamps, such as, for example, high pressure sodium (HPS) discharge lamps, typically comprises a tubular ceramic arc tube sealed at each end with a ceramic seal button. Such tubular ceramic arc tubes and seal buttons are typically fabricated from translucent polycrystalline alumina (PCA). Passing through the seal button is a feedthrough member, e.g., a tube or wire, which serves the function of electrical feedthrough and electrode holder. Feedthrough members typically comprise niobium or a mixture of metals such as, for example, niobium and zirconium. The feedthrough member and seal button component are typically held together by means of a sealing frit, normally including oxides of calcium and alumina. The sealing frit may further include oxides of barium, magnesium, boron, strontium, beryllium, and/or yttrium. The structure of a typical commercial arc tube embodying these basic features is well known in the lighting art.
The sealing frit employed in the arc tube of a high pressure metal vapor discharge lamp must have a composition which does not react with the components of the fill gas. Additionally, the thermal expansion coefficient of the sealing frit should be within certain tolerances of that of the ceramic arc tube material so that the sealing frit will not crack upon thermal cycling. For practical reasons, during sealing it is desirable to minimize the melting point of the frit.
High pressure metal vapor discharge lamps of the high pressure sodium type operate at seal temperatures of about 700.degree. C. Although such lamp has very high luminous efficiency, the color of the light output is not satisfactory for many applications. Thus, there is a need to improve the color of such HPS lamps.
Because of the low color rendering index (CRI) and color temperature of HPS lamps, much research effort has been directed to improving the color of the lamp light output. One technique for improving color has been to increase the sodium pressure of the lamp which has the effect of increasing the overall CRI. Examples of this technology for lamps with a CRI of about 60 are described by Bhalla (J. Illuminating Engineering Society, Vol. 8, pp 202-206 (1979)). These lamps only increase the correlated color temperature of sodium lamps from about 2100.degree. K. to about 2250.degree. K. This small improvement in color temperature has not been of sufficient magnitude to compensate for other disadvantages associated with this technique. Thus, the resulting lamp has not been well received in the market. Another approach has been to raise the sodium pressure still further, which raises color temperature to about 2700.degree. K., but the drop in efficacy for such a lamp is precipitous. To increase sodium pressure, the seal temperature must be increased. Sealing frits developed for this purpose are described in U.S. Pat. No. 4,501,799. These sealing frit materials have melting temperatures in excess of 1600.degree. C. Such temperatures are much greater than those of conventional sealing frit materials which have melting temperatures of about 1250.degree. C. Further, the rare earth elements included in these sealing frits cause these sealing frit materials to be more costly than standard frit materials including alkaline earth oxide components.
U.S. Pat. No. 4,409,517 issued to Van Der Sande et al. describes achieving improved color in discharge lamps employing ceramic arc tubes which include metal halide fills. To avoid the reaction of the halide components of the fill with a niobium feedthrough, Van Der Sande et al. teach applying a halide resistant coating to that portion of the upper inlead which is in contact with the lamp fill. The coating protects the in-lead from reaction with the halide vapors.
Another technique for improving the color of high pressure sodium discharge lamps is to include additional radiating elements in the fill. This technique was originally described in U.S. Pat. No. 3,521,108 issued to Hanneman. These lamps typically operate with seal temperatures about 1000.degree. C. Such lamps often experience premature failure.